Safety control circuit for forced draft



March 22, 1955 C. A. REICHELDERFER SAFETY CONTROL CIRCUIT FOR FORCEDDRAFT HEATING SYSTEMS Filed Oct. 20. 195

United States Patent SAFETY CONTROL CIRCUIT FOR FORCED DRAFT HEATINGSYSTEMS Charles A. Reichelderfer, Columbus, Ohio, assignor to NormanProducts Company, Columbus, Ohio, a corporation of Ohio ApplicationOctober 20, 1950, Serial No. 191,293

2 Claims. (Cl. 158-1) The present invention relates to heatingapparatus, and more specifically to an improved safety control circuitfor forced draft heating systems of the type embodying a fluid fuelburner, electrically-actuated valve means for controlling the flow offuel to the burner, and an electric motor driven blower or fan arrangedto forcibly circulate air across the burner to effect an elficient heattransfer within the heating system.

In the past, numerous types of forced draft heating systems embodying afuel burner and a motor driven fan or blower operable to pass air acrossthe burner have been proposed for use. One particular type of heatingsystem of this character comprises the so-called unit heater of the typenormally employed in heating relatively large areas such as factories,auditoriums, or other large rooms which are usually difiicult to heatefiiciently through a central heating plant. Generally, in previouslyknown forced draft heating systems, the fuel control valve for the mainburner of the system is arranged to be opened concurrently orsimultaneously with the energization of an electric motor employed indriving a forced air fan or blower unit arranged to circulate air pastthe burner. Various control circuits have been proposed to effect thesimultaneous opening of the fuel valve and the energization of theblower motor. In such previously known circuits, should the fan motorfail to operate by reason of various mechanical dlfilCllltlBS, theassociated fuel valve of the heating system would nonetheless be openedto supply fuel to the burner, with the result that localized overheatingof the system 'would be encountered, which in many instances proveddangerous and resulted in the destruction of the heating system.

It follows, therefore, that the primary object of the present inventionis to provide a control circuit for forced draft heating systems whichfunctions to positively prevent opening of the associated fuel supplyvalve of the burner element in the absence of a positive circulation ofair across the burner by the associated blower or fan of the system,thereby preventing destructive and dangerous overheating of the systemin the event of failure of the motor driven blower.

It is another object of this invention to provide a control system forforced draft heating systems which functions automatically in responseto temperature conditions within the zone to be heated by the system toestablish parallel operating circuits for an electrically-actuated fuelsupply valve and the motor of a blower or fan; which makes provision forenergization of the blower prior to the opening of the fuel supply valveto assure a positive flow of air across the burner prior to combustionof fuel within the system; and which further provides for continuedoperation of the fan or blower following the extinguishment of fuel atthe burner, in order that the system may be sufliciently cooled afteroperation, and that useful heat developed within the system may bedischarged to the zone being heated by the system, rather than beingretained in the heating apparatus.

For a further and more detailed understanding of the present inventionand the various additional objects and advantages realized therefrom,reference is made to the following description and the appended drawing,wherein the single figure illustrates in diagrammatic form the presentforced draft heating system and control circuit therefor.

Referring now to the drawing, the numeral 5 designates generally agaseous fuel burner which is positioned within an internal combustionchamber 6 defined by a plurality Patented Mar. 22, 1955 of walls 7disposed within an open ended casing 8. The walls 7 defining thecombustion chamber 6 are preferably positioned within the casing 8directly ahead of a rotary forced air fan or blower 9 which is drivenupon energization of an electric motor 10. Preferably, the walls 7defining the combustion chamber are spaced relative to one another toprovide a cellular construction within the casing 8 over and throughwhich a portion of the air circulated by the fan 9 may pass to be heatedand thereafter discharged through the grilled discharge opening 11 ofthe casing 8. The combustion chamber 6 is open, as at 12, adjacent thefan 9, and terminates at its opposite end in a flue gas outlet 13 fromwhich gaseous products of combustion emanating from the burner 5 may besuitably piped to the outside atmosphere. Connected with the burner 5 isa fuel supply conduit 14 in which is interposed a control valve 15 ofthe type which is opened in response to the energization of a solenoidwinding 16, and which is normally closed when the winding 16 remainsdeenergized. It should here be understood that the preceding apparatusand combination of parts are common to many types of forced draftheating systems and in and of themselves constitute no part of thepresent invention.

In accordance with this invention, a safety control circuit is providedwhich prevents the opening of the fuel valve 15 in the absence of apositive circulation of air by the fan 9, and as will hereinafter bemore fully described, provision is made for the continued operation ofthe fan 9 following closure of the fuel valve, in order that heat builtup within the system may be efficiently utilized and distributed to thezone being heated, rather than being retained within the heatingapparatus and permitted to dissipate therein.

The reference characters L1 and L2 designate the main power supply linesfor the present control circuit. Connected with the power supply linesL1 and L2, as by the leads 17, is the primary winding of a transformerT. Connected with one side of the secondary winding of the transformerT, as by the lead 18, is the thermally-responsive bimetallic spiral 19of a room thermostatic switch 20. The thermostatic switch 20 is providedwith a stationary contact 21 arranged to be engaged by the spiral 19 asthe temperature of the zone to be heated by the system falls below apredetermined temperature. The stationary contact 21 is electricallyconnected by a conductor 22 with one side of a relay switch winding 23,while the opposite side of the winding 23 is connected by a lead 24 withthe secondary of the transformer T to complete an operating circuit forthe relay winding. The winding 23 of the relay is provided with amovable armature 25 which is connected to actuate a double-pole,single-throw switch 26. Normally, the switch 26 occupies an openposition so long as the winding 23 is deenergized and is provided withthe switch blades 27 and 28 which are electrically connected with theseparate current supply lines L1 and L2. The switch 26 is provided withstationary contacts 29 and 30 which, upon energization of the winding23, are engaged by the blades 27 and 28.

Electrically connected with, and leading from, the stationary terminal29 of the switch 26 is a lead 31 which is connected to one end of thewinding 16 of the solenoidactuated fuel valve 15. The opposite end ofthe winding 16 is electrically connected with a lead 32 extending to thestationary contact terminal 33 of a normally open time delay switch 34.The contact 33 is arranged in the path of a movable thermally-responsivebimetal arm 35 which is arranged in close thermal proximity to aresistance-type heating element 36 electrically connected respectivelywith the lead 31, as at 37, and a second lead 38. The opposite end ofthe bimetal arm 35 is electrically connected through a conductor 39 witha stationary contact terminal 40 of a normally closed limit controlswitch 40a arranged within, or in close thermal proximity to, thecombustion chamber 6 of the heater casing. The limit control switchembodies a movable bimetal arm 41 whose outer end is movable to engagethe contact 40. The opposite or stationary end of the bimetallic arm 41is electrically connected with the lead 38, and in operation, the arm 41is arranged to flex outwardly out of engagement with the stationaryterminal 40 of the switch in response to a predetermined hightemperature within the combustion chamber 6. As such, the limit controlswitch functions to break the operating circuit for the fuel supplyvalve 15 in the event of overheating within the combustion chamber. Theopposite end of the lead 38 is connected with a stationary contactterminal 42 of a normally open draft-responsive switch 42a which alsocomprises a bridge 43 carried on a movable draft-responsive arm 44 whichextends within the flue outlet 13 in the path of combustion productsbeing circulated by the fan 9, and which is mounted for pivotal movementto an open position in the absence of a flow of air upwardly through theflue 13, and which is movable to a circuit closure position, as shown infull lines in the drawing, in response to the circulation of air by thefan 9 upwardly through the fiue 13. Located adjacent to, but spacedfrom, the stationary contact 42 of the draft switch is a secondstationary contact 45 which is also arranged to be electrically engagedby the bridge 43 of the draft-responsive switch arm 44. The secondterminal 45 of the draft switch is electrically connected by a lead 46with the stationary terminal 30 of the double-pole, single-throw switch26. Thus, it will be seen that the solenoid winding 16 for the fuelsupply valve 15 is disposed in series circuit with the draft switch 42a,the limit control swtich 41, and the time delay switch 34, and theopening of any one of these three switches serves to break the operatingcircuit for the winding 16 and thereby prevent the opening of the fuelvalve 15 and the flow of fuel to the burner 5.

One of the terminals of the fan motor 10 is connected with the currentsupply line L2 by the conductor 47, while the other terminal isconnected with a second lead 48 which extends to the stationary contactterminal 49 of a normally open thermally-responsive switch 50 which isdisposed within the casing 8 of the heating apparatus and which isresponsive to the temperature within the heating system to control theoperation of the fan motor 10. The switch 50 is provided with a spiralbimetallic element 51 which is electrically connected by way of a lead52 with the opposite current supply line L1. Normally, thethermally-responsive switch 50 is arranged to remain open so long as thetemperature within the heater casing 8 remains below a predeterminedrelatively low temperature, and is closed when the atmosphere within thecasing 8 reaches a predetermined high temperature slightly below thenormal operating temperatures within the casing 8 during burning of thefuel at the burner 5. Connected in bypassing relation to the switch 50is a conductor 53 which extends between the lead 31 and the second lead48 of the motor 10. Thus, an operating circuit for the motor 10 may beestablished from the current supply lines L1 and L2 either by way of theconductor 53 upon closure of the switch 26, or by way of the lead 52 andswitch 50 upon closure of the latter.

In operation, assuming the system to be cool, an operating cycle isinitiated upon the attainance of a predetermined low temperature withinthe zone to be heated by the system. The room temperature-responsivethermostat 20 closes to establish an operating circuit for the winding23 which, upon energization, closes the blades 27 and 28 to establishparallel operating circuits for the fan motor 10 and the solenoidwinding 16 of the fuel valve. Upon closure of the double-pole,single-throw switch 26, an operating circuit is established for themotor 10 by way of the leads 53, 48, and 47. Following energization ofthe motor 10, the fan 9 is driven to forcibly circulate air across theburner and in and around the walls 7 defining the combustion chamber. Asair passes from the fan 9, the same swings the movable arm 44 of thedraft switch 42a from its dotted line position as shown in the drawingto its full line position of circuit closure with respect to the contactterminals 42 and 45. Thus, current flows through the lead 46, theterminals 42 and 45, the lead 38, and through the normally closed limitcontrol switch 41. Upon closure of the draft switch, the resistanceheating element 36 of the time delay switch 34 is energized to heat thebimetal arm 35 which, with a predetermined time lag. engages thestationary contact 33 completing an energizing circuit to the solenoidwinding 16 of the fuel valve 15. Upon energization of the winding 16,the valve is opened to permit fuel to flow to the burner 5 and to bethere ignited by a suitable pilot flame, not shown. Fuel is continuouslysupplied to the burner 5 until the atmosphere of the room or zone to beheated by the system reaches a desired and predetermined hightemperature. At this time, the room temperature-responsive thermostat 20opens, thus deenergizing the relay winding 23 and opening thedouble-pole, single-throw switch 26. Upon opening of the switch 26, thewinding 16 is deenergized and the fuel valve 15 closes, thus shuttingoff the fuel supply to the burner 5. However, the fan motor 10 remainsenergized due to the closure of the thermally-responsive switch 50 solong as the temperature within the casing 8 remains above apredetermined de sired level, thus permitting the fan 9 to continuouslyoperate after extinguishment of the flame at the burner 5, in order thatthe system may be cooled by circulation of air therethrough, and inorder that more efficient use may be made of the heat within the unit byutilizing the heat built up within the system during operation of theburner to increase the temperature of the zone to be heated. After apredetermined time of operation of the fan 9 following extinguishment ofthe burner 5, the thermally-responsive element 51 of the switch 50 iscooled sufiiciently to open the operating circuit for the motor 10, thusstopping the operation of the fan 9.

The purpose of the time delay switch 34 is to provide a predeterminedand relatively short delay in the energization of the winding 16 of thesolenoid valve 15 following closure of the draft-responsive switch 42a.The provision of this time delay switch is deemed advantageous toprevent energization of the solenoid valve 15 in the event ofintermittent fluttering or oscillation of the movable arm 44 caused byup drafts through the flue outlet of the system. It will be understoodthat certain adverse weather conditions, or improper installations mightresult in periodic fluttering of the member 44 in the absence of apositive flow of air through the system established by the fan 9, and insuch event, the time delay provided by the switch 34 prevents accidentalenergization of the valve 15.

In view of the foregoing, it will be seen that the present inventionprovides an improved and mechanically efficient safety control circuitfor forced draft heating systems, wherein a motor driven fan or bloweris utilized to forcibly circulate air to the burner unit of the system.Control circuits formed in accordance with this invention arecharacterized by their ability to positively prevent the burning of fuelwithin the system in the absence of the forced circulation of airtherethrough, and in the event of back pressures or downdrafts orrestrictions within the flame outlet of the heating system, thuspreventing dangerous and destructive overheating within the unit in theevent of mechanical failure of the motor, or its operating circuit,employed to drive the fan or blower unit, and the possibility ofcombustion products being forced outwardly to the zone being heated.Further, the present control system provides for a predetermined timedelay in the opening of the fuel valve of the system following theinitial energization of the blower unit, thus assuring a ready supply offorced air across the burner prior to the ignition of fuel within thesystem. A further advantage of the present control system resides in theprovision of a thermally-responsive switch in series circuit with theblower unit by which the blower is maintained in an energized conditionfor a predetermined time following extinguishment of the burner toenable the system to be cooled and to drive off excess heat within thesystem into the atmosphere to be heated and thereby to make efficientuse of all fuel burned in the system.

It should be borne in mind that the individual switch elements, as shownand described in the appended drawing and the foregoing description, areillustrative of one form or type of switch suitable to accomplish theends of this invention, and that other suitable types of switchinstrumentalities may be employed in substitution for those specificallydescribed without departing from the spirit of the invention or thescope of the following claims.

I claim:

1. A forced draft heating apparatus embodying a fuel burner;electro-responsive valve means for controlling the passage of fuel tothe burner; a motor-driven blower arranged to circulate air to saidburner; and a control system therefor comprising, in combination: afirst switch means responsive to the temperature of a zone to be heatedby said apparatus for establishing parallel operating circuits for saidelectro-responsive valve means and for the motor of said blower; secondswitch means connected in series with said 'electro-responsive valvemeans and arranged to normally maintain the operating circuit for saidvalve means open in the absence of circulation of air by said blower; atime delay device including a switch connected in series with said valvemeans and electrically connected with said second switch means andincluding an electro-responsive actuator connected to be energizedthrough said second switch means for closing the operating circuit forsaid valve means a predetermined time following closure of said secondswitch means, whereby to delay energization of said valve means uponclosure of said first switch means; and a thermally-responsive switchconnected in series with the motor of said blower and in bypassingrelation to said first switch means and responsive to temperatureconditions within said apparatus for closing an operating circuit forthe motor of said blower independently of said first switch means, saidthermally-responsive switch being effective to maintain an operatingcircuit for the motor of said blower so long as temperatures within saidapparatus remain above a predetermined level.

2. In a forced draft heating system; a casing defining a combustionchamber having inlet and outlet ends; a fluid fuel burner positioned insaid chamber between the inlet and outlet ends thereof;electro-responsive valve means for controlling the flow of fuel to saidburner, said valve means being energizable to permit free flow of fuelto said burner; a blower arranged to positively circulate combustion airthrough said chamber from the inlet end toward the outlet end thereof;an electric motor for driving said blower; a first thermally-responsiveswitch means positioned in a zone to be heated by said systemand havingelectrical connections with said electro-responsive valve means and saidmotor and operable to open or close parallel operating circuits for saidvalve means and said motor; a draft-responsive switch including anactuating element positioned in said chamber and elec trically connectedin series with said valve means, said draftresponsive switch beingoperable in response to a positive flow of air through said chamber toestablish an operating circuit for said electro-responsive valve means;a time delay device including a switch connected in series between saidvalve means and said draft-responsive switch and including anelectro-responsive actuator connected to be energized through saiddraft-responsive switch for closing the operating circuit for said valvemeans a predetermined time following closure of said draft-responsiveswitch, whereby to delay energization of said valve means upon closureof said first thermallyresponsive switch means; and a secondthermally-responsive switch means positioned in thermal proximity tosaid combustion chamber and electrically connected with said motor inby-passing relation to said first thermally-responsive switch means andoperable in response to a predetermined temperature within said casingto maintain an operating circuit for said motor independently of saidfirst thermally-responsive switch means.

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